karwos wrote:
I'm not a thermal engineer but...
It's just approximations of course... but, when everything is working well (heatsinks, ihs) then answer is: diffrence between core and IHS, shouldn't be bigger than material conducting diffrences. Maybe I say 10 or 20%.
If i'm wrong, then let somebody correct me.
I don't have a correction, just underlining one of your qualifiers - when the IHS is working well, or correctly mounted at Intel.
Consider this thread: http://forums.anandtech.com/showthread.php?t=2261855
In particular, look at his list of links for the "Proof that the benefit from Delidding is entirely due to reducing the CPU-to-IHS gap"
This latter discussion involves comparing TIM compounds, and in that list he also shows the effect of running the CPU without the IHS (fitted with an adapter to support the HS).
The bottom line is that the primary issue is a random manufacturing flaw where the IHS isn't making contact.
So, in your equations one must account for the lack of contact, and how much resistance to thermal transfer is associated with it.
In reality thermal compounds do NOT conduct heat well on their own. When there's a gap between the core and the IHS, the compound can't transport sufficient heat away from the die. When that flaw isn't present, either because the fabrication at Intel doesn't lift the IHS off the die, or some brave enthusiast "delids" their processor, scrapes off the glue lifting the IHS and thus actually corrects the flaw, the IHS does a fairly good job.
The thread I linked claims a 19.8 C drop in full load temperature compared to the stock IHS without other interventions, when overclocked to 4.7 Ghz.
He shows a 4.7 Ghz overclock where the stock IHS (before delidding) on water (Corsair H100) hit 97.6 C, but the "corrected" IHS on the same setup maxed at 77.8 C.
He clearly shows this is independent of the TIM compound used, give or take 1 or 2 C, and entirely associated with the IHS mounting at fabrication.
It's also interesting to note his chart on bare mounting the HS, removing the IHS entirely.
For his cpu, stock speed is 3.5 Ghz. At 3.5 Ghz under full load, the stock IHS temp rose to 64.8 C, hit 56.4 C with the "corrected" IHS (after delidding), hit 55.6 without the IHS.
In other words, while temps dropped over 10 C just correcting the IHS, there was only a 0.8 C difference between the correct IHS and the bare HS mount.
At 4.7 Ghz the difference between corrected IHS and bare mounting the HS was 4.2 C. The IHS increased the highest overclock temp by 4.2 C, but the gap in the stock IHS increased that by 19.8 C.
Total potential temperature drop, from a flawed stock IHS to a bare HS mount (no IHS), overclocking to 4.7 Ghz on water (Corsair H100) was 24 C.
There's an interesting implication from the shim test described at the link above. Using a caliper, he measured the stock CPU as 4.21 mm thick, and observed that when the glue was removed under the IHS there was a reduction of 0.06 mm in thickness. His shim is 0.06 mm thick for his tests.
The implication is that we MIGHT gain knowledge about a particular CPU by measuring it's thickness and comparing that measurement in a database. My own hypothesis is that CPU's exhibiting temperature anomalies will be closer to the observed 4.21 mm than the 4.15 mm of the delidded version (meaning a 0.06mm gap was due to the glue).